Soon after the advent of the laser in 1960, its suitability was tested as a transmitter for optical radar systems. However, it was not until the advent of Q-switching in 1962 that single pulse outputs became available. During the period 1963-1964, the inventor was responsible for one of the first systematic field trials ever undertaken to assess the potential of laser-radar. These field trials were carried out by the Royal Radar Establishment, Malvern, U.K., on the army proof range in Lark Hill, Salisbury Plains and the results of the trials have been published in Royal Radar Establishment Technical Memorandum, July 1966. During these early field trials of laser radar it became clear that whilst the inherently narrow beamwidth of laser-radar had unique advantages in that relatively small targets such as distant, low flying aircraft filled the beam, leading to high transmitted energy utilization, it also became clear that such narrow beamwidths made it virtually impossible to acquire the target unless one had an accurate estimate of its coordinates via either a wide angle optical telescope or a conventional radar if one utilized prior art acquisition techniques.
On the other hand, if one utilized sophisticated laser based techniques the acquisition problem could be approached in two ways. Firstly, the laser beam could be scanned so that the scan pattern would cover a relatively large area compared with the unscanned beam. Secondly, the laser beam could be diverged The problem with the first approach was the fact that no technique existed whereby a laser beam of appropriate power could be scanned at the required rate. The problem with the second approach was the fact that no technique existed to boost the energy per unit area of the beam as it was diverged It was also realised during these early field trials that once a laser-radar had locked onto a given target, the said target could be destroyed by converging the beam. Since the destruction of the target would be carried out by the laser beam being utilized for the high precision tracking of the said target, its destruction would be fully optimised.
It was concluded that the best approach to the realisation of a practical configuration of a diverging-converging beam laser-radar system would be via the use of optical fibres. It was proposed that one end of the optical fibres should be attached to a flexible, transparent membrane several meters in diameter which would act as the surface of the optical transmitter, the remainder of which took the form of an enclosure which could be evacuated or pressurized as required. In the former case the light from the transmitter would converge and be focussed at a distance related to the radius of the concave surface. To diverge the beam the enclosure would have to be pressurized so that the membrane took the form of a convex surface. Laboratory tests were undertaken at the Royal Radar Establishment in 1964 to assess the feasibility of the concept. Unfortunately, the very first experiment, namely the study of helium-neon laser beam propagation along an optical fibre bundle showed that the output diverged by 30.degree. compared with the 0.1.degree. divergence of the input beam. It was clear that single-mode optical fibres would be required in a coherently packed bundle.
Studies carried out in Australia since that time have led to a solution to the problem. The present invention is based on the said solution. It was realised that the problem of realising a variable beam width laser-radar system depended on the optically polished output face of a coherently packed optical fibre bundle behaving as a phased array of optical transmitters. Such phased array transmitters of electromagnetic energy are well known in the microwave region of the electromagnetic spectrum where the wavelength of the transmitted radiation is about one centimeter or about ten thousand times longer than laser wavelengths. To achieve a phased array of optical fibre transmitters across the optically polished output face of a coherently packed optical fibre bundle it is necessary to delay the emergence of light from some fibre ends relative to others and to do this in a systematic way across the whole of the output aperture of the laser-radar transmitter. Techniques to achieve phased array optical scanning using an appropriately excited bundle of optical fibres have been described in a co-pending Australian patent application (No. PD 7059 lodged 8 Dec. 1978 (inventors J. L. Hughes and A. K. Ghatak)) and have also been outlined in "Applied Optics" July 1st, 1979.